This invention relates generally to semiconductor p-n junction solar cells and more particularly to semiconductor p-n junction solar cells having an electric field which extends into the semiconductor body for extracting carriers which are photogenerated within the body by the solar energy impinging upon the cell.
The advantages of the present invention will be more clearly understood when they are compared to a conventional prior art solar cell such as shown in FIG. 1. For purposes of discussion the cell is assumed to be a silicon n.sup.+p junction solar cell.
Conventional solar cells are generally constructed from a p-type wafer of silicon having resistivity in the range 0.5-5.OMEGA.-cm and thickness of approximately 200 .mu.m. Special techniques are employed in the preparation of the silicon wafer so as to maintain a minority carrier lifetime in the range of 10-100 .mu.s.
Using techniques well known to those skilled in the art a n.sup.+p junction is then formed near one surface of the wafer. For example, an n.sup.+ layer is formed by solid state diffusion, so as to create an n.sup.+p junction between the n.sup.+ layer and the p-type cell body. In state-of-the-art cells, the n.sup.+p junction is located at a depth of approximately 2000 A beneath the surface which receives the radiant energy.
An ohmic electrical contact is made to the bottom of the cell. The contact may be made by coating the bottom with an appropriate metal (e.g., aluminum or a thin layer of titanium followed by a layer of gold). A similar metal electrical contact is formed on the top of the cell, except that for the top a special pattern of contact fingers and an antireflection coating, as shown in FIG. 1, are generally employed so that the incident light can be transmitted efficiently into the body of the cell. An alloying step may also be used to reduce the contact resistance between the metal layers and the cell.
Photons of light energy from the sun enter the cell at the top and are absorbed as they pass into the body of the cell. Absorption of this light is accompanied by the creation of electron-hole pairs at various depths within the semiconductor, as shown in FIG. 2. These electrons and holes are then free to diffuse toward the contacts of the cell. As will be explained in more detail later, any given photogenerated carrier can, on average, only travel a distance L (called the diffusion length) away from the depth at which it is created before it either recombines with a carrier of opposite polarity or is trapped at a defect in the semiconductor lattice.
In order to extract electrical power from the cell, it is necessary to separate these diffusing carriers before they recombine or become trapped and supply them to a load connected between the top and bottom contacts. More precisely, the output power which can be developed by the cell shown in FIGS. 1 and 2 will be determined by the number of photogenerated electrons which can be collected into the n.sup.+ layer. The mechanisms of electron transport across the body of the cell and electron collection across the n.sup.+p junction into the n.sup.+ layer are therefore of fundamental importance to the operation of the cell.
As just mentioned, the mechanism of electron transport in the body of a conventional cell is diffusion. Thus, any electron that is generated within a diffusion length L of the n.sup.+p metallurgical junction can be transported to this junction before it recombines with a hole or becomes trapped at a defect site in the cell body. Collection of these electrons across the n.sup.+p junction into the n.sup.+ layer is then accomplished through the action of the built-in electric field which surrounds the metallurgical n.sup.+p junction.
To explain this collection mechanism, we show in FIG. 3 an expanded view of the n.sup.+p junction, in which a space charge layer is shown on the two sides of the metallurgical junction. The space charge layer is a region surrounding the metallurgical junction in which a dipole charge layer develops in the course of fabrication of the n.sup.+p junction. The doping impurities which must be introduced into the silicon to obtain p-type conductivity in the cell body will be negative in the space charge layer, while those that produce n-type conductivity will be positive in the space charge layer. These charges are shown in FIG. 3 with circles around them to indicate that they are bound into the lattice. They are not free to move; the electrons and holes generated in the cell body are free to move.
As a result of these bound charges, electric forces (or fields) are developed within the space charge layer. As indicated in FIG. 3, the direction of the electric force will be such as to attract electrons, which are negative, toward the bound positive charges on one side of the space charge layer. Holes will be attracted toward the bound negative charge on the other side. Hence, any electron which diffuses into the space charge layer from the p-type body of the cell will be pulled across the space charge layer into the n.sup.+ layer by the electric force that already exists in the space charge layer. At the same time, holes which diffuse from the p-type cell body into the space charge layer will be pushed back into the body of the cell. The electric field in the space charge layer thereof provides the mechanism for separating electrons and holes which reach the space charge layer by diffusion. This separation of electrons and holes which occurs at the pn junction is responsible for the production of electric current, and therefore electric power, in the cell.
Because the electric field produced in the space charge layer is high, the efficiency with which electrons can be transported across the space charge layer is essentially 100%. Hence, the central problem in making an efficient solar cell is to increase as far as possible the probability that an electron which is photo-generated in the cell body can reach the space charge layer. Stated more precisely, the efficiency of the mechanism by which the electrons are transported from the body of the cell, where they are created, to the edge of the space charge layer, is the major factor which determines the efficiency of a solar cell.
In conventional cells, this transport mechanism is, as mentioned earlier, minority carrier diffusion. From a practical point of view, use of the diffusion mechanism as the means for transporting photogenerated carriers to the n.sup.+p junction implies that the minority carrier lifetime in the silicon material should be as large as possible, so that the electrons that are generated deep in the p-type body can diffuse to the n.sup.+p junction before they either recombine with holes or get trapped at defect sites within the crystal.
To be somewhat more precise, it is shown in the mathematical theory of pn junction diodes, (of which a solar cell is only a special case), that if minority carriers are diffusing in a region where their recombination lifetime is .tau., then the average distance they can travel before recombining with a majority carrier is ##EQU1## where L is called the diffusion length, and D is the diffusion coefficient of the diffusing species. The diffusion coefficient D is a basic property of the material, having a value of 35 cm.sup.2 /sec for electrons in pure silicon at room temperature. The lifetime for a particular type of carrier can also be considered to be a basic material property in that it has an upper limit which can be determined from first principles. A precise analysis of the problem of ultimate solar cell efficiency has been published by Shockley et al., Journal of Applied Physics, Vol. 32, No. 3, pp. 510-519, March 1961, who show that the upper limit on carrier lifetime in silicon is on the order of 1 sec, and, as a result, the theoretical maximum efficiency of a silicon solar cell, of arbitrary construction, is approximately 25%.
However, in actual cases this upper limit on lifetime is rarely (or perhaps never) achieved because carrier lifetimes are extremely sensitive to the material processing techniques that are employed to build the cell. As an example, silicon intended for communication satellite applications is grown and processed in such a way that electron lifetimes on the order of 10 - 100 .mu.s are achieved, which result in diffusion lengths for electrons of approximately 200 .times. 10.sup.-.sup.4 cm or greater. Such cells typically have efficiencies on the order of 10-12%.
Efforts to increase this efficiency have been based on the recogition that, due to surface recombination effects, carrier lifetimes at the front and back surfaces of the solar cell can be small even though the carrier lifetime in the body of the cell may be large. Short range drift fields (so-called back-side fields) have been introduced on the bottom surface of present day cells to prevent electrons that are generated near the bottom surface from reaching that surface and recombining. A similar possibility has been suggested for the top surface of the solar cell, but the advantages to be gained from top side fields are not significant in cells that are carefully processed, and as a result, state-of-the-art cells do not currently use surface fields on the top side of the cell. An analysis of the effects of these short range fields is given in Ellis and Moss, Solid State Electronics, Vol. 13, pp. 1-24, Jan. 1970, Pergamon Press and references cited there.
As a practical matter, state-of-the-art cells fabricated using the principles just outlined have efficiencies of 17-19%. However, both the starting materials (i.e., essentially defect-free silicon crystals having very long minority carrier lifetimes) and the processing techniques required to obtain the high efficiency are very costly, so there is substantial interest in and need for a new solar cell design that is capable of producing high-efficiency cells from materials that can be made cheaply such as, for example, silicon ribbon, silicon crystals grown from metallurgical grade material, or even polycrystalline silicon plates.